Magnesium oxide is widely used as a fire brick material, catalysts, and filler for cement or paper pulp, and so on as well as for an application as a raw material for magnesium metal. In such applications, especially in the case of using for a metal to form an alloy or a material that requires corrosion resistance such as fire bricks, it is imperative that the impurities contained in the magnesium oxide for using as a material is low. For example, in the case of using magnesium oxide as a material for fire bricks, it is required to reduce calcium to a level of about 1 to 2% or less.
However, it is difficult to obtain a large amount of such magnesium oxide being low in impurity content and high in purity from natural ores such as magnesite. Accordingly, a desired oxide was often obtained by converting magnesium obtained by leach of an ore by the addition of an acid into a hydroxide or a carbonate, and then roasting it. For this reason, the ability to produce magnesium oxide is limited and magnesium oxide is very expensive.
On the other hand, it is also known that ores containing magnesium are present together with nickel oxide ore. In conventional smelting of nickel oxide ore, pyrometallurgy has often been used in which an oxide ore is put into a furnace together with a reductant and then roasted to afford nickel metal or a sulfide. However, it was not possible to use pyrometallurgy effectively because magnesium is allowed thereby to form an oxide together with impurities and is separated as a slug.
In recent years, hydrometallurgy called HPAL process, in which nickel oxide ore is leached under high temperature and pressure conditions by using sulfuric acid, has also been used. In an HPAL process, nickel oxide ore is put into a pressurizable vessel together with sulfuric acid and then polyvalent metal such as nickel is leached into a sulfuric acid solution under high temperature and pressure atmosphere at about 250° C. It is characteristic in that use of the HPAL process makes it possible to leach nickel at a high efficiency from low quality nickel oxide ore containing nickel in a content of about 1 to about 2% by mass or less.
The leached nickel is solid-liquid separated from a slurry containing a leach residue with continuous addition of a neutralizing agent and then is separated from impurities by addition of a neutralizing agent as described in Patent Literature 1. Moreover, the leached nickel forms a precipitate of a sulfide by the addition of a sulfidizing agent and is separated from components which are not intended to be recovered, such as aluminum, manganese, and magnesium, and thereby is refined into an intermediate material for obtaining nickel metal or a nickel salt.
On the other hand, the solution resulting from the separation of nickel and containing components which are not intended to be recovered is transferred to effluent treatment and then discharged via treatments such as neutralization. In other words, magnesium was not effectively utilized as a resource even in the HPAL process of Patent Literature 1.
While limestone, calcium hydroxide, or the like of high industrial availability is used in a large amount as a neutralizing agent in smelting using a wet process described above, calcium sulfate formed by neutralization also forms a precipitate. This led to increase in the amount of the precipitate and raised such problems as increase in labor required for securing a disposal place, in disposal costs, and besides, increase in environmental impact.
Use of a highly soluble salt as a neutralizing agent is conceivable as a method for inhibiting the amount of a precipitate from being increased by a neutralizing agent, and the above-described magnesium oxide and magnesium salts such as magnesium hydroxide are suitable for this application. For this reason, in hydrometallurgy for nickel oxide ore, attempts have also been made to recover magnesium from waste water generated in steps and use magnesium oxide as a neutralizing agent.
Specifically, one example of the methods for recovering magnesium from a solution to be transferred to effluent treatment is the method described in Patent Literature 2. In Patent Literature 2 is proposed a process of recovering magnesium oxide from a source of magnesium sulfate, said process including the steps of: providing a source of magnesium sulfate in solution that is derived from part of a process associated with the leaching of a metal containing ore or concentrate; converting the magnesium sulfate in solution into solid magnesium sulfate; contacting the solid magnesium sulfate with elemental sulfur in a reducing atmosphere; and recovering the magnesium as magnesium oxide, and the sulfur as sulfur dioxide gas.
In Patent Literature 3 is proposed an atmospheric leach process in the recovery of nickel and cobalt from lateritic ores, said processing including the steps of: separating the lateritic ore into a low magnesium containing ore fraction, and a high magnesium containing ore fraction by selective mining or post mining classification; separately slurrying the separated ore fractions; leaching the low magnesium containing ore fraction with concentrated sulfuric acid as a primary leach step; and introducing the high magnesium content ore slurry following substantial completion of the primary leach step and precipitating iron as goethite or another low sulfur containing form of iron oxide or iron hydroxide, wherein sulfuric acid released during iron precipitation is used to leach the high magnesium ore fraction as a secondary leach step.
It is expected that the use of the methods described in Patent Literatures 2 and 3 makes it possible to use magnesium contained in nickel oxide ore as a neutralizing agent or to recover magnesium from a neutralized liquid and iteratively use it as a neutralizing agent.
In the case of using these methods, however, a huge amount of heat energy is required for concentrating magnesium from a large amount of waste water and there is a concern that impurities contained in ores accumulate in the process with iterative use of a neutralizing agent.
Moreover, normally, the content of magnesium contained varies and is not stable depending upon the type, mine site, and mine time of ores. For this reason, if magnesium is insufficient, combined use of a conventional calcium-based neutralizing agent that is inexpensive and capable of being supplied stably, such as calcium hydroxide, limestone, or the like, is conceivable. In this case, however, calcium is also brought into the process and is circulated within the process as in the above-described conventional methods. Moreover, an attempt to recover magnesium from waste water will result in failure of magnesium to be used for applications other than a neutralizing agent because some calcium behaves in the same manner as magnesium.
Examples of methods for separating magnesium and calcium in a solution from each other include the method described in Patent Literature 4. In the method described in Patent Literature 4, magnesium hydroxide is recovered from waste fluid containing a large amount of magnesium sulfate disposed and discharged in a flue gas desulfurization plant in which magnesium hydroxide is used as a desulfurizing agent and is recycled to a flue gas desulfurization step. Thus, the method is conducive to recycling and environmental cleanup. Specifically, ammonia is added to flue gas desulfurization waste water containing magnesium sulfate to form and settle magnesium hydroxide, and then lime milk is added to the resulting liquid to form calcium sulfate and ammonia and the ammonia is circulated between steps. If the thus-obtained magnesium hydroxide is slurried with the final waste fluid of the present process and circulated to a desulfurization plant, then perfect circulation of desulfurization plant waste water is realized and it becomes possible to eliminate disposal and discharge of waste water. Moreover, the resulting calcium sulfate can enhance advantage in its external sale by providing a washing step to increase its purity.
However, since ammonia is handled in the method of Patent Literature 4, there are problems with the method that a complicated facility is required and that investments and operating costs are increased. Therefore, it was difficult to handle it easily.
As described above, it was not easy to obtain magnesium oxide being low in impurity content and high in purity efficiently at a low cost by such conventional methods.